1. Field of the Invention
The present invention relates to a process for the preparation of hydrocarbon mixtures having a content of alkyl tert-alkyl ethers or alkylene bis-(tert-alkyl ethers) by catalysed reaction of a crude hydrocarbon mixture with lower alkanols or alkanediols on a catalyst. Crude hydrocarbon mixtures which can be employed are those having a sulphur content of up to 1500 ppm, of which up to 500 ppm are present in the form of mercaptans, disulphides or a mixture of the two. The sulphur-resistant catalytic agent employed here is an ion exchanger having SO.sub.3 H groups, which is charged with Ni, Pd or a mixture of a metal from a first group of Pd, Ru, Rh and Pt with a metal from a second group of Fe, Co, Ni, Cu, Ag and W.
The invention also relates to this catalytic agent.
In the process according to the invention, the content of highly unsaturated compounds furthermore is largely eliminated by simultaneous addition of hydrogen. Isomerization of the terminal monoolefins furthermore takes place in the process according to the invention. All these reactions effected by the catalytic agent are impaired only to a considerably reduced extent by the sulphur content of the crude hydrocarbon mixture.
2. Description of the Related Art
It has been known for a long time that light benzines and naphtha can be catalysed purely thermally or catalytically, ethylene and propylene being obtained in the desired manner; these two olefins are chiefly further processed to the corresponding polymers. In addition, such cracked products comprise further alkenes, which may be straight-chain or branched, alkanes and aromatics. For working up these products, the mixture which remains after the ethylene and propylene have been separated off are divided into distillation cuts, which in general include the range of the same carbon number or adjacent carbon numbers. The tert-alkenes present in these distillation cuts can be reacted with lower alkanols on acid catalysts to give alkyl tert-alkyl ethers. These ethers can be isolated, and are sought after solvents and octane boosters in carburettor fuels.
However, it is also possible to leave such etherified tert-alkenes in the hydrocarbon mixture and to add such a mixture to the carburettor fuel as an octane booster without further working up. In such a case, it is desirable to eliminate so-called gum-forming agents. Gum in connection with fuels is understood as meaning a content of oligomeric or polymeric substances in the fuel which occurs as an evaporation residue on analysis of the fuels. The presence of gum in fuel leads to coking and deposits in the combustion chamber of the engine and is therefore undesirable. Gum-forming agents are highly unsaturated compounds, for example diolefins and/or acetylene compounds. According to EP 197 348 B1, simultaneous etherification of the tert-alkenes and removal of the gum-forming agents by their conversion into alkenes is achieved by treating corresponding distillation cuts of such crude hydrocarbons simultaneously with methanol and hydrogen over a catalyst which comprises a commercially available cation exchanger (styrene/divinylbenzene copolymer having sulphonic acid groups) in the H.sup.+ form which has been charged with palladium.
It has furthermore been found (EP 338 309 A), that it is possible additionally to carry out partial isomerization of alkenes over the ion exchanger/Pd catalysts described. Thus, for example, in the particular distillation cuts, 1-butene is rearranged into 2-butene, which is desirable for alkylations; in the distillation cuts of higher carbon number, isomerization of branched olefins is observed, in particular, such that those in which the double bond is not on the branching carbon atom are converted into those which are then tert-alkenes and thus contribute to further etherification.
It has now been found that the hydrogenation activity for elimination of the gum-forming agents, but in particular the desirable isomerization activity, of catalysts of the cation exchanger/platinum metal type, in particular those of the cation exchanger/Pd type, decreases prematurely under the influence of a high sulphur content in the crude hydrocarbon mixture and falls to a level which can no longer be utilized industrially. To eliminate such an undesirable influence by a sulphur content on catalytic reactions, it has been necessary to date to carry out a separate pretreatment of the crude hydrocarbons over an inorganic, sulphur-resistant contact catalyst or to employ another sulphur-resistant catalyst for the catalytic reaction, which, however, can no longer have the effect of the desired multiple functions like the cation exchanger/platinum metal type. Another possibility is to treat the sulphur-containing feed with a "scavenger", for example a resin carrying basic groups (anion exchanger), beforehand. This possibility entails in costs, like all additional treatments, but is only suitable for sufficiently acid S compounds: for example, mercaptan compounds are bonded, but thioether or disulphides are not.
It was therefore desirable to discover a process and suitable catalytic agents for this, which are sulphur-resistant and nevertheless can have the effect of the multiple functions described, whereby a cumbersome and cost-intensive separate removal of sulphur should be eliminated at the same time.